Hi @TimBaker,
Following your example, and imagining custom parameter X on both Custom Forms A and Custom Form B, if custom Form B is deleted, all X-entered data that was ONLY on Custom Form B will be deleted, and all X-entered data that was ALSO or ONLY on Custom Form A will be preserved, untouched.
That said, being permanent, this is also an important concept to practice in your preview environment so you understand the risks and mechanics.
Regards,
Doug